Does the POP3 protocol allow you to specify a subset of emails to download?

Question

I am writing a POP3 mail client. I want to leave the messages on the server, but I don't want to have to redownload all messages every time I reconnect.

If I download all the messages today, and reconnect tomorrow does the protocol support the ability to only download the messages from the last 24 hours or from a certain sequential ID? Or will I have to redownload all of the messages again?

I am aware of the Unique IDentification Listing feature, but according to http://www.faqs.org/rfcs/rfc1939.html it's not supported in the original specification. Do most mail servers support this feature?

Yes, my client supports IMAP too, but this question is specifically for the POP servers.

Answer 1

The way I have seen that handled in the past is on a client-by-client basis. For example, if I use Scribe to get e-mail on one machine without deleting, then move to another machine, all e-mails are downloaded again despite the fact that I've seen them before. Internally, I imagine the client has a table that stores whether or not an e-mail has been downloaded previously.

There's nothing in the protocol that I'm aware of that would allow for that.

Answer 2

Have you considered using IMAP?

Answer 3

Sort-of. You can download individual messages, but you can't store state on the remote server.

See the RETR command at http://www.faqs.org/rfcs/rfc1939.html.

Answer 4

I've done it.

You'll have to reread all the headers but you can decide which messages to download.

I don't recall anything in the header that will give you a foolproof timestamp, however. I don't believe your solution is possible without keeping a record of what you have already seen.

(In my case I didn't care--I was simply looking for messages with certain identifying features in the header--those messages were downloaded, processed and killed, everything else was untouched.)

I also wonder if you're misunderstanding the protocol. Just because you download a message doesn't mean it's removed from the server. It's only removed from the server if you give an explicit command to kill the message. (And when a message contains so many attachments that the system time-outs before you properly log off and thus your kill command is discarded you'll be driven up the wall!) (It was an oversight in the design. The original logic was attach one file over 100k, or as many as possible whose total was under 100k. Another task barfed and generated thousands of files of around 100 bytes each. While it was a perfectly legit, albeit extreme, e-mail nothing was able to kill it!)

Thus if I were writing a mail client I would simply download anything I didn't already have locally. If it's supposed to remain on the server, fine, just don't give the kill command.